In recent years nanomaterials have become a subject of significant interest due to their size-dependent properties and potential application in the field of catalysts, sensors and other fields of nanotechnology and microelectronics. Nanoparticles of metals or their alloys with diameter ranging from several to several dozens of nanometers are of particular interest. Metallic nanoparticles display increased reactivity of the surface atoms and exceptional optical properties. Moreover, because of their highly developed surface area, nanoparticles are considered an attractive material for use in heterogenic catalysis. Metallic nanoparticles can also be used in medicine as carriers for therapeutic substances.
Different methods of nanoparticle synthesis are known. One of such methods utilizes continuous-flow microreactors. A continuous-flow microreactor is a device used for carrying out a chemical reaction in a small volume. It comprises one or more channels of small diameter for transporting the reagents in and the product out of the system, as well as a mixing compartment or a combination of mixing compartments, which allow for the mixing of the reagents. Use of such type of a device has many advantages, as it allows for quick and thorough mixing of the reagents and makes the temperature control easy. The continuous-flow microsystems for nanoparticle synthesis have been described, for example in publication WO 2004/076056 and WO 2009/133418. Use of continuous-flow microsystems for synthesis of nanoparticles of noble metals, such as Pt, Pd and Au, has been discussed in a review article by Wojnicki M. et al. (“Zastosowanie mikroreaktorów przepywowych do syntezy nanoczstek metali szlachetnych (Pt, Pd, Au). Przegld literatury”, Rudy Metale, 2011, vol. 56, no 12, p. 745-752).
Methods for nanoparticle synthesis using continuous-flow microreactors comprise chemical reduction of metal precursors, i.e. metal salts, such as Pd(OAc)2, PdCl2, PtCl4, AuCl3, AgNO3, Cu(OAc)2, CuCl2, RhCl3 and FeCl3, in a solution and in the presence of surfactants or other organic molecules, which aim to stabilize nanoparticles (their concentration and type determine nanoparticle size) and prevent nanoparticle aggregation.
Sodium citrate, ascorbic acid, hydrazine, sodium borohydride, lithium tetraethylborohydride, methyl alcohol, ethylene glycol, 1,2-hexadecandiol and glucose are commonly used as reducing agents (see, for example Streszewski B. et al., “Synteza nanoczgstek zota metod redukcji jonów kompleksowych zota (III) za pomoc hydrazyny w ukadzie mikroreaktora przepywowego”, Rudy Metale, 2011, vol. 56, no 12, p. 752-761).
Polyvinylpyrrolidone (PVP) (as discussed in, for example, in Wagner J. et al., “Generation of metal nanoparticles in a microchannel reactor”, Chemical Engineering Journal, 2004, no 101, p. 251-260; Köhler J. M. et al., “Formation of isolated and clustered Au nanoparticles in the presence of polyelectrolyte molecules using a flow-through Si chip reactor”, Journal of Materials Chemistry, 2005, no 15, p. 1924-1930; Wagner J. et al., “Continuous synthesis of gold nanoparticles in a microreactor”, Nano Letters, 2005, vol. 5, no 4, p. 685-691.) and polyvinyl alcohol (PVA) (as discussed in, for example, in Köhler J. M. et al., “Formation of Au/Ag Nanoparticles in a Two Step Micro Flow-Through Process”, Chem. Eng. Technol. 2007, vol. 30, no 3, p. 347-354; Köhler J. M. et al., “Preparation of metal nanoparticles with varied composition for catalytically applications in microreactors”, Chemical Engineering Science, 2008, vol. 63, p. 5048-5055; Wojnicki M. et al., “Synteza nanoczstek zota stabilizowanych PVA (alkohol poliwinylowy) w mikroreaktorze przepywowym”; Rudy Metale, 2009, vol. 54, no 12, p. 848-852) are often used as stabilizing agents. Other stabilizing agents used in the art include sulfobetaine (Song Y. et al., “Synthesis of palladium nanoparticles using a continuous flow polymeric microreactor”, Journal of Nanoscience and Nanotechnology, 2004, vol.4, no 7, p. 788-793) and poly(benzyl ether) (Torgoe K. et al., “Microflow reactor synthesis of palladium nanoparticles stabilized with poly(benzyl ether) Dendron ligands”, Journal of Nanoparticle Research, 2010, vol. 12, no 3, p. 951-960). Sometimes a reducing agent can, at the same time, play a role of a stabilizing agent, as it is in the case of sodium citrate (Weng C. H. et al., “Synthesis of hexagolan gold nanoparticles using a microfluidic reaction system”, Journal of Micromechanics and Microengineering, 2008, vol. 18, p. 1-8; Sung-Yi Yang, “Size controlled synthesis of gold nanoparticles using a micromixing system”, Microfluid Nanofluid, 2009, vol. 8, p. 303-311).
Straightforward size control of the produced nanoparticles is an advantage of using continuous-flow microsystems for nanoparticle synthesis. Nanoparticle size depends on temperature, reagent flow rate and length of the channels, where reduction reaction takes place. Use of surfactants and other organic molecules as stabilizing agents is a disadvantage of nanoparticle synthesis methods known in the art. Use of stabilizing agents is very unfavorable, because they strongly adsorb on the surface of the newly formed nanoparticles and before further use of the nanoparticles it is necessary to use complex procedures, for example electrochemical methods, to purify their surface (see, for example, publication of Solla-Gullon J. et al., “Electrochemical characterization of platinum nanoparticles prepared by microemulsion: how to clean them without loss of crystalline surface structure”, J. Electroanal. Chem., 2000, vol. 491, no 1-2, p. 69-77), and this, very often, alters the nanoparticle properties (for example, catalytic properties). Furthermore, very often it is not possible to remove the adsorbed stabilizing agents entirely from the nanoparticle surface (see, for example, Park, J. Y. et al., “The Role of Organic Capping Layers of Platinum Nanoparticles in Catalytic Activity of CO Oxidation”, Catalysis Letters 2009, vol. 129, no 1-2, p. 1-6).
A further disadvantage related to use of continuous-flow microsystems known in the art is that they are limited to low reagent concentrations, which results in obtaining of colloids having low concentration of nanoparticles.
Therefore there exists a considerable demand for a method enabling nanoparticle synthesis, in particular nanoparticles of metals or their alloys, in essentially pure form, which method would make it possible to use such nanoparticles directly, i.e. without the need of performing the inconvenient purification processes. It would be also advantageous, if a method was developed, whose application would lead to obtaining stable nanoparticle colloids of high concentration and would make it possible to produce nanoparticles on a large scale.